Prosthetic materials are engineered elements which can achieve biological function when placed within a living organism. An important class of prosthetic materials are those which are used to repair and replace human body tissue such as osseous matter. To replace biological tissue in an acceptable, long lasting manner, the replacement materials must join with the surrounding living matter. Proper melding is achieved through the use of an appropriate material having a micro-network of capillaries permeating the structure to permit living tissue in-growth.
Such porous networks must be continuous, permitting unrestricted passage of blood and other body fluids from the surrounding tissue while also providing structural support. This can be easily envisioned in the design of artificial bone wherein osseous replacement materials must support the forces and stresses associated with the skeletal system and simultaneously allow passage of blood gases, nutrients, waste products and other extracellular material to and from the surrounding tissue.
In reconstructive surgery such as repair of highly comminuted fractures, healing can be accelerated by inclusion of materials having such porous matrix adjacent to the break point to enhance bone growth. Rebuilding of damaged long bones can also benefit from insertion of such porous prosthetic materials to re-achieve the desired pre-damage shape and strength.
Such porous yet semi-rigid materials are found in nature. For example, spiny starfish, certain sea urchins and coral exhibit a solid structure formed of calcium carbonate having a network of interconnecting pores and significant void volume in the form of a micro-porous matrix. Specifically, the slate pencil sea urchin has cigar-shaped protrusions that have a void volume of 50 percent and a porous structure with pore diameters of approximately 25 .mu.m. Certain coral provide similar attributes with pore diameters of approximately 250-600 .mu.m.
In the past, these aquatic materials were used to form biologically acceptable structures such as through hydrothermal treatment of the calcium carbonate skeletons to form hydroxyapatite. More detailed discussion of such techniques may be found in U.S. Pat. Nos. 3,890,107, 3,929,971, 4,231,979, 4,722,870 and 4,861,733, the teachings of which are all incorporated by reference herein.
Although these procedures offer a unique class of structures, they are accompanied by several significant drawbacks. The naturally forming aquatic structures are never completely uniform and often exhibit imperfections detrimental to surgical implantation. In addition, the materials are expensive to harvest, and such gleaning of nature has raised environmental impact concerns in some quarters.
These problems have led to a search for techniques to engineer and manufacture porous materials having specifically delineated structural properties in a controlled manner. For instance, U.S. Pat. Nos. 5,348,788 and 5,455,100 (the teachings of which are incorporated herein by reference) describe such porous articles having very specific, delineated structural properties referred to, in concept, as "minimal surface structures."
However, a problem has been recognized with minimal surface structures when used in orthopedic surgery/prosthetic applications. Minimal surface structures in three-dimensional arrays are "too regular" and not well received by orthopaedic surgeons. Technical difficulties in fabricating the ideal minimal surface structure include the exact registration of successive layers and the requirement of alternately placing successive layers "front-to-front" then "back-to-back." Thus, the known art still requires the exact registration of successive layers of two-dimensional sheets in un-natural, alternating layers to form a three-dimensional material which is too uniform and too regular for use as practical substitutes for natural bone.
U.S. Pat. No. 5,487,933 (the teachings of which are incorporated herein) covers the fabrication of a two-dimensional sheet structure that represents the basis for fabrication of an entirely new synthetic bone substitute material. The following description includes several important advancements which represent the next generation, three-dimensional bone-substitute material.